1. Field of the Invention
The present invention relates to power hoists or lifts and more specifically to a low-profile, compact hydraulic hoist for dump truck bodies.
2. State of the Prior Art
There are a variety of known styles and structures of hoists for dump truck bodies. One common hoist style that has gained wide spread popularity and acceptance over the past three decades is known as a "scissors hoist", the configuration and action of its frame and lift arms being reminiscent of a pair of scissors. The popularity of the common scissors hoist, a typical example of which is illustrated in FIG. 2, has been due primarily to a combination of attributes, including its relative stability, general compactness when folded so that it is generally not visible externally when collapsed to the down position, its effective operation with one hydraulic cylinder, and its overall cost effectiveness and reliability. Essentially, as shown in FIGS. 2 and 3, the common scissors hoist 1 mounts on top of the truck chassis frame C, under the floor or deck D of the dump body B, and between the two longitudinal dump body frame members LL and LR.
In spite of the popularity of the conventional scissors-type hoist, there are also a number of well-known disadvantages associated with its use. For example, while it is fairly compact with respect to other, previously known hoist configurations, it still cannot be made compact enough to fit between the truck chassis C and the body B without having to cut out one or more cross sills S, as shown in FIG. 2, and still be able to handle normal loads.
The primary reason for this limitation is illustrated in FIG. 4, in which the conventional scissors-type hoist is shown in its collapsed position, i.e., with the truck body B in the normal, unraised condition. In this illustration, the weight W of the body B and any load contained therein acts vertically downward through upper mount pin 2 at the distal end of upper frame member 1. The lift force F applied by the hydraulic cylinder 3 is applied to load pin 4 positioned above a line 5 that extends through cylinder mounting pin 6 and scissors hinge pin 7. The lift force F is applied to load pin 4 in the direction of a line 8 extending through cylinder mounting pin 6 and load pin 4. Therefore, the lift force F can be resolved into its vertical and horizontal components F.sub.v and F.sub.h, respectively, acting on load pin 4 as a function of the angle .theta. between line 8 and horizontal. In the collapsed condition shown in FIG. 4, the line 5 is substantially horizontal; therefore, the angle .theta. is between lines 5 and 8.
As illustrated in FIG. 4, therefore, the lift force F can be resolved into a vertical component F.sub.v =F sin .theta., and into a horizontal component F.sub.h =F cos .theta.. In solving the moments about hinge pin 7, EQU W.times.d.sub.1 =(F.sub.v .times.d.sub.2)+(F.sub.h .times.h)(1)
or EQU W.times.d.sub.1 =(Fsin.theta..times.d.sub.2)+(Fcos.theta..times.h)(2)
Therefore, solving for lift force F required to lift the weight W, ##EQU1##
Consequently, if the load pin is moved closer to line 5, the force F required to lift weight W increases dramatically. Hence, the load pin 4 has to be kept a practical height h above line 5 in order for the hoist 1 to work under normal conditions and with normal loads. This requirement in practice prohibits the load pin 4, thus the height of the hoist 1, from being reduced enough to avoid having to cut out cross sills S of the body B when the hoist is being mounted on the truck T. The disadvantages of cutting out cross sills S include possible compromises in the structural integrity of the body B, increased labor costs for hoist installation, and the like.
A modified scissors-type hoist 10, as shown in FIG. 5, commonly known as the "drop hinge" type of scissors hoist, was developed to solve these problems. The drop hinge scissors hoist 10 still has the same basic structure with the load pin 4 positioned above a line 5 extending between cylinder mounting pin 6 and hinge pin 7 to get the required mechanical advantage. However, the upper frame member 9 and lower frame member 12 have a downwardly bent "dog leg" portion 11 that extends below the top of chassis C, so that hinge pin 7 can be positioned some distance below the top of the chassis C, thus lowering line 5 substantially. With line 5 lower, load pin 4 can also be lowered enough to keep the lift arm or upper frame member 9 below the cross sills S while still maintaining the required mechanical advantage. Thus, the cross sills S do not have to be cut or modified to accommodate the hoist 10.
While the drop hinge scissors hoist 10, as described above, solved some of the problems encountered with the conventional scissors hoist 1 by dropping a portion of the hoist and the hinge below the top of the chassis C, that solution is no longer as attractive as it was several years ago. The problem now is that truck chassis are not only being built more compactly than in previous years, but more factory-installed equipment, such as exhaust components, catalytic converters, mufflers, tail pipes, fuel systems component, and the like are being packed by the truck manufacturers into the limited space between the chassis frame members. Thus, when the time comes to install a hoist in the after market, very often there just is not sufficient space left between the chassis frame members to accommodate the "dog leg" or drop hinge part 11 of the hoist 10.
There is, therefore, a growing need for a still more compact, minimum profile scissors-type hoist that can fit almost entirely in the space above the chassis frame, between the longitudinal frame members of the body, and below the cross sills, yet which has sufficient mechanical advantage to lift the weight of the truck body with a normal-sized load therein. Prior to this invention, there was not any hoist available that could meet these requirements.